Abstract : For licensing purposes, safety cases of nuclear power plants (NPPs) must be presented at the Regulatory Authority with the necessary confidence on the outcomes of the models used to analyze the plant safety behavior. In the present work, we consider the problem of providing a quantitative indication of the confidence in the safety margin estimation by a model with uncertain inputs giving in output the maximum outlet water temperature of the residual heat removal system (RHRs) in accident scenarios of the high temperature reactor-pebble modular (HTR-PM). The quantitative evaluation is carried out by means of a computational procedure of literature, based on order statistics (OS). The procedure is analyzed with respect to some of its key parameters defining the sample size and the number of uncertain inputs.